My ten year multiple entry Canada visa dated 20th Feb.2020 is valid upto 15th January, 2021 upto the date of expiry of my passport. I have renewed my passport in December 2020. I want to travel to Canada in May, 2021. Can I travel on the old visa in my old passport with renewed passport? If not, do I need to reapply for visa?
valid upto 15th January, 2021 upto the date of expiry of my passport
Assuming this means that your passport and Canadian visa will expire on January 15, 2021, you would need to apply for a new visa.
Usually Canada doesn't issue a visitor visa beyond the validity of the passport. If the visa is expired, you cannot enter with it (with certain exceptions for study/work permit holders visiting US/St Pierre et Miquelon). You need to apply for a new visitor visa, but usually without too much difficulty barring significant change of circumstances (keep the COVID situation in mind, though).
If you have a valid (unexpired) visa in an old passport, you may use it with a new travel document.